The invention relates to an improved patellar bearing for a prosthetic knee joint that will reduce or eliminate the sudden posterior rotation xe2x80x9cclunkxe2x80x9d of the patella (knee cap) with its implant, especially in knees with low patellar position in relation to the joint line.
When, due to disease or injury, the articulating surfaces of a knee joint become sufficiently disabling and painful (arthritic), these surfaces are commonly replaced using a surgical operation, either in whole or in part, by prosthetic implants. The most commonly performed operation of this kind is the replacement of all of the joint surfaces in the knee, referred to as a total knee replacement. Sometimes the damage is limited to only one side of the knee joint, either the medial (the side toward the midline of the body) or the lateral (the side away from the midline of the body) compartment. Then, the operation to replace just these damaged surfaces between the femur (thigh bone) and the tibia (leg bone) is referred to as a unicompartmental replacement. A unicompartmental replacement can be either medial or lateral. Sometimes the damage is limited to the joint surfaces between patella (knee cap) and the femoral groove with which the patella articulates. In this case, the operation to replace these joint surfaces is called a patellofemoral replacement.
A multitude of different designs of total or partial knee replacement systems have evolved over time using various metals, plastics, and ceramics as the bearing surfaces. Naturally, all these bearing surfaces are subject to wear giving rise to failure of the bearing, fracture of the bearing, or loosening of the implant over time. When this happens, another operation is required to replace or revise the implant(s). This is referred to as a revision replacement. Because this is a second operation performed on already altered bone, tendons, ligaments, and muscles comprising the joint, the results are not as good compared to the initial or primary joint replacement operation. Therefore, an important common goal of all joint replacement systems is to provide a comfortable, functioning joint that lasts for the life of the patient.
The total knee replacement prosthesis design that apparently has the best-reported longevity along with the lowest reported complication rate involving the patella is the design by Buechel and Pappas, U.S. Pat. Nos. 4,309,778 (1982), 4,340,978 (1982), and 4,470,158 (1984). Among the design features of this patellar prosthesis that have contributed to its superior longevity and low complication rate are: (1) the deeper (compared to other patellar prosthetic designs), V-shaped contours of the posterior articulating bearing that resists displacement or dislocation from the femoral groove with which it articulates; (2) the broad, congruent bearing surfaces providing area contact loading (as opposed to dual point contact or line contact loading as in other patellar prosthetic designs) that reduces contact stress and thus reduces wear; and (3) the provision for the plastic patellar bearing to rotate on a cone-shaped trunion of the metal base plate causing this rotating patellar bearing to act in a self-aligning manner and thus further reduce stress, wear, and loosening. This patellar prosthesis is known as the low contact stress, or LCS, rotating patella.
FIGS. 1A and 1B show a total patellofemoral implant employing a prior art LCS patellar bearing 11 implanted into the patella 12. A trochlear component 13 is shown implanted in the femur 14. FIG. 1A shows the knee flexed at about 45xc2x0. FIG. 1B shows the knee flexed to about 120xc2x0. It is seen that the patella, with its implanted prosthesis, suddenly rocks, arrows 16, over the sharp and angular superior edge 17 of the LCS patellar bearing.
This feature of the prior art LCS rotating patellar bearing is deleterious, detrimental and disadvantageous. When this patellar prosthesis is implanted into a patient either for a total knee replacement or for a modular total patellofemoral replacement (see my U.S. patent application Ser. No. 09/148,110 filed Sep. 4, 1998) and the patient has a low riding patella, as referenced from the joint line between the femur and the tibia, an annoying and painful symptom described by patients as a xe2x80x9cclunkxe2x80x9d will likely occur. Patients with patellas positioned lower than average are said to have xe2x80x9cpatella inferaxe2x80x9d, a congenital or acquired condition. The opposite condition is referred to as xe2x80x9cpatella altaxe2x80x9d. The aforementioned annoying and painful xe2x80x9cclunkxe2x80x9d starts occurring when the patients with patella infera have regained their normal range of motion after surgery. As the patient flexes (bends) the knee beyond a right angle, the patella that is already low to begin with slides farther around the curvature at the lower end of the femur and reaches a point where it suddenly rocks back over the sharp and angular superior edge of the plastic patellar bearing. This sudden posterior rotation of the patella and its implanted bearing occurs around a transverse axis of the patella and is felt by the patient as a sudden, annoying, and painful xe2x80x9cclunkxe2x80x9d.
The sharp, angular superior edge of the aforementioned patellar prosthesis bearing also has the potential to damage the normal articular cartilage in the knee when the prosthesis is implanted for a total patellofemoral replacement. When a total patellofemoral replacement operation is performed, the entire patellar articular surface is replaced, but only the anterior groove on the front of the femur (the trochlea) is replaced, not the entire articular surface on the lower end of the femur. The remaining normal cartilage on the end of the femur is purposely left intact to articulate with the remaining normal cartilage on the upper end of the tibia. After a total patellofemoral replacement, when the patient flexes (bends) the knee beyond a right angle, the patellar prosthesis moves farther around the curve of the lower end of the femur progressively leaving its contact with the metal trochlear implant and making greater and greater contact with the remaining normal cartilage. The sharp and angular superior comer of the LCS prosthetic patellar bearing can press into, gouge, or damage the softer normal articular cartilage.
An improvement in the aforementioned patellar bearing to eliminate or reduce the sudden, annoying, and painful symptom experienced by patients as a xe2x80x9cclunkxe2x80x9d is beneficial and desirable. Said improvement would also lessen the risk of damage to the normal articular cartilage when the patellar bearing is used for a total patellofemoral replacement.
The objects and advantages of the present invention are:
(a) to reduce or eliminate the sudden posterior rotation xe2x80x9cclunkxe2x80x9d experienced by certain patients who have had the Low Contact Stress (LCS) patellar prosthesis implanted during total knee replacement or total patellofemoral replacement, and
(b) to reduce or eliminate articular cartilage damage caused by the sharp and angular superior edge of the aforementioned patellar prosthesis when it has been implanted during a total patellofemoral replacement.
The improved patellar bearing has an extended superior-to-inferior dimension with rounded contours on the superior and inferior edges of its joint surface that articulates with either a femoral prosthetic component or a trochlear prosthetic component.